runtime: unblock SIGUSR1 for TestPreemptM

TestPreemptM tests signal delivery using SIGUSR1, but (for unknown
reasons) SIGUSR1 is blocked by default on android/arm and
android/arm64, causing the test to fail.

This fixes the test by ensuring that SIGUSR1 is unblocked for this
test.

Updates #10958, #24543.

Change-Id: I9f81fbab53f96c74622aabcb6f5276f79e2b6d33
Reviewed-on: https://go-review.googlesource.com/c/go/+/203957
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/export_unix_test.go

@@ -33,6 +33,10 @@ var waitForSigusr1 struct {
 // the ID of the M the SIGUSR1 was received on. If no SIGUSR1 is
 // received for timeoutNS nanoseconds, it returns -1.
 func WaitForSigusr1(ready func(mp *M), timeoutNS int64) (int64, int64) {
+	lockOSThread()
+	// Make sure we can receive SIGUSR1.
+	unblocksig(_SIGUSR1)
+
 	mp := getg().m
 	testSigusr1 = func(gp *g) bool {
 		waitForSigusr1.mp = gp.m
@@ -46,6 +50,8 @@ func WaitForSigusr1(ready func(mp *M), timeoutNS int64) (int64, int64) {
 	waitForSigusr1.mp = nil
 	testSigusr1 = nil
 
+	unlockOSThread()
+
 	if !ok {
 		return -1, -1
 	}